This invention relates to metals such as aluminum and to metal ingot, slab, strip or billet and the like. More particularly, it relates to improving the as-cast surface quality of metal ingot, slab, strip, bar or billet and the like to provide cast material having substantially improved metal surface suitable for further processing or fabricating.
During the casting of metal (aluminum) ingot, slab, strip, and other forms, various defects may appear on the surface. These defects can be topographical, solidification related, and contaminant related. Many of these defects develop during the early stages of solidification and within a thin surface layer of solidified metal known as the "shell".
Topographical defects include surface roughness caused by the mold or mold coating material. During the early stages of solidification, the shell is in direct and intimate contact with the mold. If the mold surface exhibits a particular topography, this topography is replicated on the shell and thus causes a surface defect. Solidified metal can also collect on sections of the mold and/or metal entry devices such as nozzles and inscribe striations in the shell. These types of defects are often referred to as "drags".
Solidification related defects include as a condition known as "liquation" wherein cooling of the solidifying shell within the mold results in local contraction of the shell. Contraction occurs in most solidifying metals because the density of solid metal is usually greater than in the molten state. As the surface of the shell retreats from the mold wall, it creates a gap. Local heat transfer is reduced, reheating occurs, and the surface of the shell advances back in the direction of the mold. Heat transfer is again increased, invoking a subsequent retreat. This cyclic process causes undulations in the surface of the shell, and segregation of solute elements in the case of an alloy. The problem is particularly exacerbated by wide freezing range alloys, or alloys with a substantial fraction of eutectic liquid remaining at a temperature corresponding to the eutectic reaction isotherm. If present, secondary phases in the alloy, such as iron aluminides, may also be non-uniformly distributed on the surface of the solidifying shell, as a result of this process. Poor feed metal flow conditions into the mold can also result in solidification related surface defects.
Contaminant related defects are caused by oxides, salts, and combinations of oxides and salts present in the molten metal that form a layer on the shell during solidification. Since these contaminants are immiscible, they readily segregate to the surface. The well known "oxide patches" in the direct chill (DC) casting of magnesium containing aluminum ingot are an example of contaminant related defects.
Several methods directed at preventing surface defects have been disclosed in the literature. Electromagnetic casting is one example. Improved metal flow distribution facilitated by novel nozzle design in continuous slab and strip casting is another example. All of these attempts to solve surface problems are classified as preventative.
Scalping of ingot is a commonly employed remedial practice to establish a surface suitable for rolling. The rolling faces of an ingot are essentially milled, off-line, to depths of 0.25 inch (or greater), prior to hot rolling. Ingot scalping is a capital intensive process that can only be performed on the rolling faces of the ingot. Since the edge surfaces of an ingot are contoured to control stresses during rolling, conventional scalping is not a viable alternative to remove edge defects. The presence of oxide patches on the edges of an ingot constitutes a problem that requires careful hot rolling practices to avoid edge cracking.
Often, it is desirable to remove such defects while the metal is in a hot condition, for example, shortly after casting, particularly in continuous casting systems. However, this has the problem that the metal is plastic, making the treatment thereof difficult. For example, mechanical brushing or burnishing has been found unsatisfactory to remove such defects. Thus, there is a great need for a process to remove the defects relented to, preferably on a continuous basis, to provide a defect-free surface suitable for rolling to provide sheet or plate or like material with a high quality surface.